Named after the Buddhist divinity of wisdom, Monju, located in Japan's Fukui prefecture, is Japan's only fast-breeder reactor. Unlike conventional reactors, fast-breeder reactors, which “breed” plutonium, use sodium rather than water as a coolant. This type of coolant creates a potentially hazardous situation as sodium is highly corrosive and reacts violently with both water and air.

On December 8th, 1995, 700 kg of molten sodium leaked from the secondary cooling circuit of the Monju reactor, resulting in a fire that made headlines across the country. Although the accident itself did not result in a radiation leak, many argue that the fire came close to breaching Monju, a catastrophe which would have spilled plutonium into the environment.

Following the fire, officials at the government-owned Power Reactor and Nuclear Fuel Development Corporation (PNC), operators of Monju, first played down the extent of damage at the reactor and denied the existence of a videotape showing the sodium spill. Later, they released still shots only, showing things like intact pipes and clean floors and claiming that there had only been “a minor leakage in the secondary sodium loop [that had] caused some fumes”. While short videos were released, these were edited to hide the full extent of the damage. Further complicating the story, the deputy general manager of the general affairs department at the PNC, Shigeo Nishimura, 49, jumped to his death the day after a news conference where he and other officials revealed the extent of the cover-up.

Starting from September of 2007, Nishimura's family brought the story back to light in a trial against the PNC at Japan's High Court.